1. Field of the Invention
The present invention relates to an illumination optical system that illuminates an image display element homogeneously, and an image display optical system and a projection type image display optical system that use the illumination optical system.
2. Description of the Related Art
In recent years, there has been a demand for a compact projection type image display apparatus that projects an image having an improved brightness.
FIG. 11 shows the configuration of a conventional projection type image display apparatus. Referring to FIG. 11, white light emitted from a light source 301 is reflected by a reflector 302, passes through a fly-eye lens 303, and is reflected by a mirror M301. Then, the light passes through a fly-eye lens 304, a polarization converting element 305 and a condenser lens 306 and the like, and then enters a dichroic mirror DM301.
Of the white light incident on the dichroic mirror DM301, light component in the blue band is reflected by the dichroic mirror DM301 and light component in the green and red bands pass through the dichroic mirror DM301. A halogen lamp, a metal halide lamp, or an ultra high-pressure mercury lamp and the like is generally used as the light source.
The light component in the blue band, reflected by the dichroic mirror DM301 that has a spectral transmittance shown in FIG. 12(A), passes a negative lens 307B, redirected by 90 degrees by a reflection mirror M302. Then, the light component in the blue band passes through a field lens 308B and enters an image display element 309B. The light component is the blue band is modulated with an image (liquid crystal image) formed by the image display element 309B.
The modulated light component in the blue band enters a dichroic prism 310, which redirects the light component by 90 degrees to an object lens 311.
The light components in the green and red bands that have passed the dichroic mirror DM301 passes a negative lens 307G and then enters a dichroic mirror DM302 having the spectral transmittance shown in FIG. 12(B). As is clear from FIG. 12(B), the dichroic mirror DM302 has the property that it reflects light component in the green band. Therefore, the dichroic mirror DM302 reflects the light component so that the light component is redirected by 90 degrees to enter an image display element 309G through a field lens 308G. The light component in the green band is modulated with an image formed by the image display element 309G.
The modulated light component in the green band enters the dichroic prism 310 and then a projection lens 311.
The light component in the red band that has passed the dichroic mirror DM302 passes through a trimming filter TR0 having a spectral transmittance shown in FIG. 12(C), a condenser lens 312, a relay lens 313, a field lens 308R, and reflection mirrors M303 and M304, then enters an image display element 309R. The light component in the red band is modulated with an image formed by the image display element 309R.
The modulated light component in the red band enters the dichroic prism 310, which redirects the red component by 90 degrees so that the red component to enter the projection lens 311.
In this manner, image light components of the respective colors combined by the dichroic prism 310 are projected onto a projection surface, for example, a screen by the projection lens 311.
With the aforementioned projection type image display apparatus, light having substantially homogeneous intensity illuminates the image display element. The area of homogeneous intensity is referred to as illumination area hereinafter. The illumination area is larger than the display area of the image display element so as to keep a margin of the illumination area with respect to the display area of the image display element due to the tilt of mirrors and eccentricity of the lenses, and to allow for shrinkage of the illumination area due to differences in curvature among the lenses and the distances between the lenses.
However, if the margin of the illumination area is too larger than the display area of the image display element, light which is not actually projected onto the screen increases, and therefore the image on the screen has less brightness. For this reason, an adjusting mechanism is provided to move the lens upward and downward and leftward and rightward and to tilt the mirrors, so that the illumination area can be adjusted to a minimum necessary size.
For the aforementioned projection type image display apparatus incorporating a plurality of image display elements, the positional errors of the illumination areas with respect to the respective image display elements need to be corrected.
For example, with a projection type image display apparatus as shown in FIG. 11 is provided with adjusting mechanisms: an adjusting mechanism that moves the condenser lens 306 upward and downward and leftward and rightward to correct the positional errors of the illumination areas of the light component in the green band, an adjusting mechanism that moves the negative lens 307B upward and downward and leftward and rightward to correct the positional errors of the illumination areas of the light component in the blue band, and an adjusting mechanism that moves the relay lens 313 upward and downward and leftward and rightward to correct the positional errors of the illumination areas of the light component in the red band. By using these adjusting mechanisms, the positions of the illumination areas are adjusted.
As mentioned above, the adjusting mechanism is required for each color of light and therefore the conventional art presents the problem of large size, poor assembly efficiency, and increased costs of the apparatus.
FIG. 13 shows a configuration of another conventional projection type image display apparatus. Referring to FIG. 13, a beam of white light is emitted from a light source 401 in the form of, for example, a halogen lamp, a metal halide lamp, or a ultra high-pressure mercury lamp or the like, and a portion of the flux of white light enters fly-eye lens 403 and 404 directly and the remaining portion of the flux of white light is reflected by a reflector 402 and enters the fly-eye lenses 403 and 404, so that the flux of white light is divided into a plurality of light flux portions. The plurality of light flux portions are polarized in the same direction by a PS conversion element 405, and then pass a condenser lens 406 and a field lens 407. This causes the light flux to have a substantially homogeneous intensity distribution. Thereafter, the light flux enters an image display element 408 in the form of, for example, a LCD and the like.
The image display element 408 forms an image in accordance with an input signal thereto and modulates the light, which has entered the image display element 408, with the image, so that the modulated light is projected by the projection lens 409 onto a screen, not shown.
The brightness of the image projected from the projection type image display apparatus greatly depends on the balance of the incidence angle of the illumination light that is incident on the image display element 408 and the F-number of the projection lens 409.
For the image display apparatus shown in FIG. 13, the incident angle of the illumination light that is incident upon the image display element 408 is determined substantially by the diameter of the condenser lens 406 and the distance D between the condenser lens 406 and the image display element 408.
If the distance D is made shorter in an attempt to miniaturize the apparatus, the incident angle of the illumination light incident upon the image display element 408 becomes large, so that a large portion of luminous flux cannot enter the pupil of the projection lens 409, decreasing the brightness of projected image.
One way of solving this problem is to dispose a concave lens between the condenser lens 406 and the image display element 408 as proposed in Japanese Patent Laid-Open No. 2000-98488 and Japanese Patent Laid-Open 2000-241882.
This method provides a shorter distance between the condenser lens and the image display element without increasing the incidence angle of the illumination light incident upon the image display element 408. Thus, this method provides a bright display image and a miniaturized apparatus.
However, as mentioned in the above publications, when the condenser lens is convex on its both sides, the apparatus is prone to outward coma aberration in the illumination area on the image display element 408 as shown in FIG. 14.
The amount of light becomes small at the four corners of the screen where the image height is largest. In order to ensure homogeneous intensity of light illuminating the image display element, the illumination area should be made larger. This decreases the amount of light that is not projected onto the screen, decreasing the brightness of the image projected onto the screen.